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Fig. 2.14 VLBI system
encodes these data by rewriting signals and necessary information in a special
format. Next, the formatted data are recorded on dedicated tapes in specialized
format by the magnetic tape recorder. It should be noted that these 14 frequency
converters have 14 independent LO signals, which will result in phase drift. So,
phase calibration is required. A phase calibration system consists of a pulse
generator, which transmit an impulse and inject it into the signals every micro-
second. This pulse injection point is defined as the reference point of the delay.
4. Finally, the observed data recorded by the magnetic tape recorder are sent to the
related processors. The processors playback the data and input them to the
correlators of corresponding channels to carry out cross-correlation computa-
tions and acquire the related function value, i.e., the interferometric fringe. After
that the computer uses its software system to obtain the required observed value
of the time delay and the time delay rate by fringe fitting computation.
The Technique of Space VLBI
To improve the resolution of VLBI, the concept of space VLBI was proposed in
1970 and the establishment of the space VLBI system was also considered. By
1980, space VLBI became more mature in theory and technical realization. In 1997,
the first space VLBI satellite (VSOP) in human history was successfully launched
in Japan. Although space VLBI was proposed for astrophysics research, conceptu-
ally it has more advantages over ground-based VLBI for application in fields like
geodesy. Therefore,
it will become a more effective geodetic observation
technology.
In light of the VLBI principle, there are no differences between space and
ground-based VLBI. A space VLBI station can be seen as a component of a
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